This invention relates to underwater photography, and more particularly to an improved underwater camera jig for use in taking stereoscopic photographs of underwater surfaces for use in analyzing the extent and character of marine growth on the surface, condition of surface coatings, and degree of pitting of hull plates, and the like.
Underwater photography has long been used as a valuable tool in determining the condition of underwater surfaces of ship hulls, bridges, piers, and other underwater structures. U.S. Pat. No. 631,222 to D. Mason provides an early example of photographic apparatus for such purposes. Commercially available portable underwater cameras and light sources that can be carried by free swimming divers have resulted in more widespread use of underwater photography.
Close-up photography, in particular, requires accurate lens to object distance control in order to assure proper focus and underwater photography, in general, can be severely hampered by turbid water conditions. Accurate distances to surfaces or objects being photographed underwater with close-up lenses is usually attained by means of a stand-off frame or bracket formed of heavy wire and attached to the camera, while the problem of turbid water has been approached by providing a clear water chamber between the camera lens and the object to be photographed, or through use of apparatus including pumps, hoses, and the like for displacing turbid water with a flow of clear water. U.S. Pat. No. 2,396,267 to E. R. F. Johnson provides examples of those expedients. Such devices as have been available, however, have been unweildy for a free swimming diver to manage, and hence the extent of use has been limited.
While conventional single image photographs can convey a great deal of visual information about the condition of an underwater surface, they provide only two dimensional views of objects and conditions that are three-dimensional in character and are significantly related, for example, to problems of fuel consumption of, and structural failures in ships.
This has been recognized in U.S. Pat. No. 4,008,606 to H. R. Talkington, wherein the hull surface to be photographed is illuminated by light falling at a predetermined angle so that shadows will be cast by discontinuities on or in the hull surface. While some information is gained in this manner as to size, other information is lost in the shadows.
Through the use of the present invention, it has been found that stereoscopic, or dual image, photographs can provide considerably more information concerning the extent and height of marine growth, size and depth of pitting of metal plates, degree of blister development in coatings, and the like. Now, stereoscopic measurements and interpretations require the making of dual photographic images with a great deal of precision, particularly as to the inter-lens distance as well as to the lens to object distance. Although stereoscopic cameras are available having spaced lenses for taking the dual images simultaneously of objects or scenes above water, and devices are known for using a conventional single lens camera for taking the dual images successively to make stereoscopic photographs in air, none of those devices, however, are suitable for conveniently carrying out close-up stereoscopic photography underwater, with turbid conditions, by free swimming divers.